Many network-connected devices are capable of receiving electrical power necessary for their operations via their data communications connections with a network. For example, power over Ethernet specifications such as IEEE 802.3af and IEEE 802.3at allow a device to receive power from a network, in addition to data communications, via one or more Ethernet ports. This allows devices to be connected to the network and powered using the same cable. Because these devices do not need a separate connection with a power source, the costs and infrastructure required for installing and maintaining these devices are minimized.
Power over Ethernet (and generally other network power systems) typically include two classes of devices. Power sourcing equipment (PSE) supply electrical power to the network powered devices (PD), which receive electrical power from the network. Devices may act as PSE or PD independently of their role in sending or receiving data. Any type of device connected with the network and a power supply may act as a PSE and any type of device connected with the network may act as a PD to receive some or all of its required electrical power via the network.
The electrical power supplied by a PSE is typically limited to prevent overloading of PSE devices and overheating of the network cable. For example, IEEE 802.3af specification compliant PSE ports (on Ethernet switches and the like) provide for 12.95 W maximum usable power at the secondary side of a PD circuit (at the client product). This specification includes allowances for the PSE output voltage being at the low end of the specification and the cable between the PSE and PD being the maximum length of 100 m of CAT-3 wire with worst case heating loss (due to cable DC resistance).
Furthermore, most PD devices are designed to limit their power consumption to the maximum power allowed by the specification. For example, PD's following the IEEE 802.3af standard limit their power consumption to less than the 12.95 W maximum allowed by the standard.
However, most PSE ports put out a voltage at or near the high end of the specification and the cable between the PSE and PD is usually nowhere near 100 m long, and is most likely to be CAT-5 or CAT-6 wire, which has much lower loss per unit length than CAT-3.